The present invention relates generally to resistors and, more particularly, to power resistors used for periodically dissipating large amounts of electrical energy over short time periods.
Resistors are typically identified and characterized by their resistance value, tolerance, and power dissipation capability. Of these, both resistance and tolerance are substantially independent of the conditions under which the resistor is used. However, the resistor""s power dissipation capability, or power rating, is specifically directed toward continuous, steady-state conditions and, while a resistor may have the ability to handle transient power surges in excess of its power rating, relatively little work has been done to develop resistors that are designed for applications involving small, steady-state currents with large transient energy surges.
One such application is in electric vehicles which typically utilize a high-voltage bus of approximately 360 volts to power various vehicle systems, including an inverter used to drive the three-phase motor that provides operating power to the vehicle wheels. Usually, the high-voltage bus includes a capacitor bank of, for example, 15,000 xcexcF, with these capacitors being charged from the high-voltage supply. The capacitor bank filters out transient voltages and helps reduce ripple on the high-voltage bus. To prevent destructively high currents during charging of these capacitors, a high-wattage current-limiting resistor is often used, with its resistance being limited to a relatively small value due to the large capacitance of the capacitors and the relatively short time in which they must be charged. For example, for a high-voltage supply of 360-400 volts, a capacitor bank of 15,000 xcexcF, and a 200 ohm current-limiting resistor (which provides three time constants of capacitor charging within nine seconds), approximately 1,200 Joules of energy must be transferred via the resistor. Due to the exponential current profile of the circuit, over half of this energy is transferred within the first second, resulting in an initial peak power surge of 800 watts with over 600 watts of power being dissipated by the resistor in the first one and one-half seconds of charging. Once charged, there is minimal steady-state current flow through the resistor with some electric vehicle applications including a bypass switch that removes the current-limiting resistor from the circuit altogether once the capacitors are charged.
Although this electric vehicle high-voltage bus application requires periodic dissipation of energy on the order of 600 to 800 watts, use of a resistor at this wattage rating is not practical due to space and cost considerations. Rather, a common alternative to using a single, high-wattage resistor is to employ two or more lower wattage resistors which together dissipate the total power supply by the high-voltage battery. For example, in a typical application such as described above, the required current-limiting resistance has been implemented using two 100 ohm, 50 watt, wire-wound aluminum cased-resistors connected in series. However, the size and termination geometry of these resistors still make it impractical for circuit board mounting of the resistors inside the environmentally sealed high-voltage enclosure that is typically used in these electric vehicle applications. Accordingly, the resistors must be mounted externally, necessitating expensive manual assembly utilizing mounting screws, flying leads, and heat-shrinkable tubing for high-voltage isolation. Although a larger number of lower wattage resistors could be utilized to permit printed circuit board mounting of the resistors inside the environmentally sealed enclosure, such an arrangement has been found to be undesirable as well, as it can require considerable space and can greatly increase the overall part count.
Apart from conventional carbon and wire-wound resistors, there exists a number of other power resistor design configurations. For example, some manufacturers offer customized printed circuit boards that can include thick film power resistors printed directly onto the printed circuit board surface. One such customizable printed circuit board is available from FERRO-ECA Electronics Company of Erie, Pennsylvania and includes a ceramic-coated metal core substrate having a printed circuit that can include pre-printed thick film resistor elements. However, these commercially available boards have been found to be unsuitable for the high-voltage bus current-limiting application discussed above since they are designed for somewhat lower power applications and are limited to a maximum power dissipation of 100 watts per square inch.
Accordingly, it is a general object of this invention to provide a resistive component that is capable of withstanding periodic, short-duration high-wattage peak power surges. Preferably, it is also an object of this invention to provide such a resistive component that is relatively compact in size and that can be mounted to a printed circuit board.
In accordance with the present invention, there is provided a resistive component that includes a generally planar substrate with a resistive element located on each side of the substrate and a plurality of terminals for connecting opposing portions of each of the resistive elements to an electronic circuit. The resistive elements have substantially equal dimensions and resistive properties such that they have substantially equal resistance values and exhibit substantial equal current densities for any given applied voltage. Preferably, the substrate is a metal or other core material having a high thermal conductivity with a ceramic or other electrically insulating layer on each side of the core material. The resistive elements can be silk-screened onto the substrate and preferably have a substantially uniform thickness such that each of the resistive elements exhibit a uniform current density when subjected to an applied voltage.
With this configuration, thermal bending of the resistive component due to differential thermal expansion at one of the substrate""s insulating layers is substantially offset by thermal bending due to differential thermal expansion at the other insulating layer. This permits design of a resistive component having a small physical size and a relatively low steady-state wattage rating, but with the ability to handle short duration, high-wattage power surges.